Needless to say I suffered hugely as a manager until I learned the difficult lesson that (as my coach at the time said to me) "You need to find new ways of being important."

不用说，当时我作为一位经理吃了不少苦头，后来才在这些苦头中领悟到(就像当时我的导师对我说的那样)：“你得找到新的方法来体现自己的重要性。”

youdao

In a sense, this is the first difficult lesson of the class.

这是这门课第一个较难的结论

耶鲁公开课 - 博弈论课程节选

I've learned a hard and difficult lesson considering the childhood environment that I was brought up in - not to discriminate against any plant or creature.

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Still, the most important lesson from the trial may be that it is incredibly difficult to tease out the long-term effects of a medicine.

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They say they learned that difficult lesson in two of the last three years, when hail destroyed most of their crops.

CNN: farmer plows